1. Field of the Invention
The present invention relates to a technique for providing a connection service to terminals each accommodated in private branch exchanges (PBXs) through a virtual private line in Integrated Services Digital Network (ISDN).
2. Description of the Related Art
There has been proposed a technique of establishing a virtual private line as necessary in ISDN (see Japanese Patent Application Unexamined Publication No. 9-266509). More specifically, PBXs are connected to each other through an analog/digital private line and, if necessary, a virtual private line can be established between the PBXs using an IDSN line. Therefore, telephone terminals accommodated in respective PBXs are capable of communicating with each other using the virtual private line as an alternative route.
Another virtual private line connection technique using ISDN has been disclosed in Japanese Patent Application Unexamined Publication No. 4-165892. In this conventional system, each PBX is connected to the ISDN through a primary rate interface (23B+D) trunk circuit. When a virtual private line is established, one or more B channel is used for voice and another B channel is used for signalling. A D-channel controller is connected to the primary rate interface trunk circuit to control the D channel of the primary rate interface. A D-channel controller is also connected to the primary rate interface trunk circuit to control the signalling B channel of the virtual private line. More specifically, SS7 (common channel signalling system No. 7) messages are controlled by the D-channel controller to be transferred on the signalling a channel of the virtual private line. Therefore, after having established the voice and signalling B channels, the existing ISDN signal procedure can be performed with no problems by conforming to the SS7 procedures.
However, it is necessary for each PBX to be provided with the D-channel controller, resulting in great difficulty in cost reduction and the increased amount of hardware in the PBX.
Further, at least two B channels are needed to establish the virtual private line. Therefore, the efficiency of use of the network is reduced and the communication cost is increased because of a charge for at least two B channels.
An object of the present invention is to provide a virtual private line connection system and method allowing efficient and economical virtual private line service in the ISDN.
According to the present invention, a method for establishing a virtual private line between a first private branch exchange (PBX) and a second PBX through an integrated Services Digital Network (ISDN) to connect a first terminal accommodated in the first PBX and a second terminal accommodated in the second PBX, includes the steps of: at the first PBX having a first digital interface trunk circuit connected to the ISDN, a) generating an SS7 (common channel signalling system No. 7) message in response to a call originated by the first terminal; b) generating an ISDN message having the SS7 message included therein; and c) sending the ISDN message to the second PBX on a D channel of the first digital interface trunk circuit, and at the second PBX having a second digital interface trunk circuit connected to the ISDN, d) receiving the ISDN message from the first PBX on a D channel of the second digital interface trunk circuit; and e) reading the SS7 message from the ISDN message received to establish the virtual private line between the first and second PBXs, allowing the first and second terminals to communicate with each other through the virtual private line.
The step (b) preferably includes the steps of: b.1) generating an ISDN message for the SS7 message; b.2) setting a virtual private line flag indicating that the SS7 message is used for virtual private line connection; b.3) storing the virtual private line flag and the SS7 message in a facility information element of the ISDN message.
The step (e) preferably includes the steps of: e.1) reading the facility information element from the ISDN message received from the first PBX; e.2) determining whether the virtual private line flag included in the facility information element is set; e.3) when the virtual private line flag is set, reading the SS7 message from the facility information element to establish the virtual private line.
As described above, the SS7 message is included in the ISDN message and is sent to the other PBX on the D channel to establish the virtual private line in the ISDN. Therefore, the need for a signalling B channel is eliminated. Since the ISDN message having the SS7 message encapsulated therein is sent on the D channel, the virtual private line can be established conforming to specifications for inter-PBX signalling protocol defined in the international standard ISO/IEC11582, ETS 300 239, JS-11582 Private Integrated Services Network (Generic Functional Protocol for the support of supplementary services). Therefore, the transparency and versatility of the public ISDN can be ensured.